A number of schemes for maintaining toner concentration can be found in the prior art. One class of device, to which the present application pertains, is called a patch sensing device. In devices of this class, the optical density of a toned test patch is used as a measure of toner concentration. If this optical density is found to be too low, toner is added to the developer station.
A particularly good device of this class is shown in U.S. Pat. No. 4,178,095 from which FIG. 2 of the present application is taken. This patent is hereby incorporated by reference for the purpose of indicating the xerographic background of the invention, and as illustrative of the state of the art.
A light emitting diode (LED 33 of FIG. 2) is electrically energized at a low level when its reflected light is used to "look at" bare, untoned photoconductor, and is electrically energized at a higher level when its reflected light is used to "look at" a less-reflective toned test patch. The level of higher LED energization is factory-adjusted such that photocell 34, which views these two photoconductor areas, detects substantially the same intensity reflected light when toner concentration is correct. As exemplary correct toner concentration is 1 wt. % toner in a toner/carrier mix which constitutes 100 wt. %.
This patent discloses a potentiometer P1 whose adjustment is operable to attain the correct toner concentration control point by way of a change in the magnitude of said higher LED energization. For example, if this level of energization is increased, more light is reflected from a given optical density toned test patch. As a result, the control apparatus operates to add toner to the developer mix, and future addition of toner occurs at a higher optical density which is caused by the higher toner concentration control point.
The prior art recognizes that the optical density of a toner image, be it on the photoconductor itself or after the toner image has been transferred from the photoconductor to copy substrate such as paper, is a function of three variables; i.e. (1) the toner concentration in the developer's mixture of toner and carrier beads, (2) the image voltage carried by the photoconductor, this in turn being a function of initial photoconductor charge magnitude, and (3) the magnitude of development electrode bias voltage, for example the voltage applied to the magnetic brush roller of a magnetic brush developer.